Images taken by NASA show a comet 91 million miles from Earth breaking apart into dozens of pieces, some as small as a house.
Astronomers say these are the clearest images of this process taken to date and could provide clues as to what causes a comet to die.
The first of the two images was captured by the Hubble Space Telescope on April 20 and shows the comet had fragmented into 30 pieces. The second, taken on April 23, shows 25 pieces. Scientists are unsure as to why the number of fragments differed between days.
"Their appearance changes substantially between the two days, so much so that it's quite difficult to connect the dots," David Jewitt, professor of planetary science and astronomy at the University of California, Los Angeles, and leader of one of two teams that photographed the doomed comet with Hubble, said in a statement.
"I don't know whether this is because the individual pieces are flashing on and off as they reflect sunlight, acting like twinkling lights on a Christmas tree, or because different fragments appear on different days."
The comet—C/2019 Y4 (ATLAS)—was first discovered on December 29, 2019, and continued to brighten until mid-March, when it suddenly started to dim. Until that point, some had predicted it would shine so bright as to be visible from Earth.
Astronomers suspected this sudden turn was due to the fragmentation or disintegration of the nucleus at its core—a process confirmed when around three pieces of the comet were photographed by an amateur astronomer, Jose de Queiroz, on April 11. Images taken later in the month revealed as many as 30 separate fragments.
According to Quanzhi Ye from the University of Maryland, College Park, leader of a second Hubble observing team, comet fragments are normally too dim to make out, which makes events on this kind of scale an infrequent occurrence—they only happen once or twice per decade.
But while the scale of this event may have been extraordinary, fragmentation itself could actually be quite common. The researchers suggest it may even be the prevailing cause of death for the icy nuclei, which is made of primordial ice and dust and lies at the heart of the comet.
Jewitt told Newsweek that if comets did not decay, they would last around 500,000 years after entering the vicinity of Earth. If nothing else happened they would eventually hit a planet or the Sun, or be ejected from the solar system, he said.
"Instead, they last maybe a ten or twenty thousand years, because of the loss of ice and occasional breakups like the one ATLAS is experiencing," he explained. "So, breakups are common astronomically speaking, but not on a human timescale."
The rapid speed at which this happens makes the process hard to decipher and astronomers do not know what causes the fragmentation to take place in the first place. According to one hypothesis, it could be triggered by the sublimation of the icy core as it approaches the Sun, when the heat causes solid material to turn to vapor. The vapor is discharged from the comet, causing it to spin. The theory is that if it spins with enough force and speed, the comet may break apart.
"Comets are in a sense somewhat fragile," said Dr Robert Massey, the Deputy Executive Director of the Royal Astronomical Society in the U.K.—describing them as agglomerations of rock and icy substances. As they travel towards the Sun, they move from the cooler regions of deep space to much warmer regions around the Solar System's central star.
"The characteristic thing we see is the gas coming out of the nucleus. That gas is often swept out by the solar wind and the radiation you get from the Sun and you get the tails that we see," Massey told Newsweek.
"If you take something that is only loosely bound together to begin with. Then it's not unreasonable to imagine it's going to break up."
Massey described the breakup as "the death of a comet."
"You're not looking at something terribly unusual," he said. "Although in this case, it is a particularly good view."
The team hopes Hubble's data will help determine whether or not this is the case, improving our understanding of what causes comets like these to break up. According to astronomers, the nucleus pre-fragmentation would have been no larger than the length of two football fields.
At the time the last observations were made, the comet was around 91 million miles from Earth. It is expected to make its closest approach on May 23, when, if any of it still exists, it will fly past approximately 72 million miles from the Earth. It will pass the Sun eight days later.
According to Jewitt, C/2019 Y4 (ATLAS) would have had a long but uneventful life in the most distant region of the Solar System, the Oort Cloud—a spherical shell made of space debris that surrounds the rest of the system. After 4.5 billion years in the Oort Cloud it has been propelled across the Solar System.
"Suddenly, it's been thrust into the hot zone near the Sun and the stress of the new environment is causing it to disintegrate," said Jewitt. "It is quite special to get a look with Hubble at this dying comet."
As for what will happen to what is left of C/2019 Y4 (ATLAS)—"The fragments will continue along the same orbit as the original nucleus but most will fade away to invisibility, losing their ice or, in some cases, disintegrating further into tinier and tinier pieces, then into dust," Jewitt told Newsweek.
This article has been updated with comments from Dr. Robert Massey and additional comments from Prof. David Jewitt.
